Known piezoelectric ceramics exhibit piezoelectricity, whereby application of an electric field generates mechanical strain and stress. Such piezoelectric ceramics are used in various types of piezoelectric elements including actuators, piezoelectric buzzers, sounding bodies, sensors and the like.
Actuators employing piezoelectric ceramics characteristically allow high precision to be obtained for fine displacement and produce large generated stress, and they are used for positioning of precision machine tools and optical devices. The compositions for piezoelectric ceramics used in actuators are, in most cases, made of lead zirconate titanate (PZT) which has excellent piezoelectric properties. However, because lead zirconate titanate contains a large amount of lead, concerns have been raised in recent years regarding the effects on the environment by elution of lead caused by acid rain. A demand therefore exists for a piezoelectric ceramic composition with a satisfactorily reduced lead content, as a substitute for lead zirconate titanate. Lead-free piezoelectric ceramic compositions have therefore been proposed to meet this demand (see Patent document 1).
Barium titanate (BaTiO3) is known as a typical lead-free piezoelectric ceramic composition with a perovskite structure. Because it has insufficient piezoelectric characteristics, however, other piezoelectric ceramic compositions are being investigated. Alkali niobates such as K0.5Na0.5NbO3 (KNN) are being studied for use, as piezoelectric ceramic compositions that can realize relatively high piezoelectric characteristics near ordinary temperature (see Non-patent document 1).
It is expected that high piezoelectric characteristics can be obtained by using such piezoelectric ceramic compositions as piezoelectric ceramic materials in ordinary devices. However, alkali niobates with high piezoelectric characteristics usually have an orthorhombic-tetragonal phase transition temperature near room temperature, and when they are used in environments with thermal cycles that straddle the crystal phase transition temperature, repeated crystal phase transition alters the domain structure of the ferroelectric substance and lowers the piezoelectric characteristics (see Non-patent document 1).